R-13 Variation of C l max for NACA 23012m and NLF 0414 Airfoils With and Without an Upper Surface 0.25-inch, Forward-Facing, Quarter-Round Simulated Ice Shape at Various Chord Positions (Re = 1.8x106, M = 0.185). (See Reference R5.) R-22
R-14 Drag Increase for NACA 23012m and NLF 0414 Airfoils With a 0.25-inch, Forward-Facing, Quarter-Round Simulated Ice Shape Located on the Upper Surface at Various Chord Positions (Re = 1.8x106, M = 0.185). (See Reference R5.) R-23
R-15 Reynolds and Mach Numbers Effects on Lift and Drag for a NACA 23012 Airfoil With and Without Simulated Ice Shape 290. (See Reference R2.) R-25
R-16 Comparison of Computed Flow Fields Using Reynolds-Averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) Navier-Stokes CFD Codes (NLF 0414 airfoil at an AOA of 7˚ and a Reynolds number of 1.8x106 with a
0.034 k/c simulated ice shape horn.) (See Reference R8.) R-29
R-17 Time-Dependent Vortex Shedding (Vorticity Contours) From an NLF 0414 Airfoil With a 0.034 k/c Simulated Ice Shape Horn Calculated Using a DES Navier-Stokes CFD Code. (See Reference R8.) R-29
R-18 Vorticity Contours for 2D and 3D DES Navier-Stokes Computational Fluid Dynamics Code Analysis of the Flow Field Around a NACA 23012m Airfoil With a k/c = 0.0139 Forward-Facing, Quarter-Round Simulated Ice Shape at the Same Time Instant (Re = 1.8x106). (See Reference R8.) R-30
R-19 Correlation of Full-Model (55 cm NACA 0012 Airfoil) and Half-Model Ice Shapes by Holding the Full Model K0, Ac, n, I, and Weh Constant. (See Reference R17.) R-33
R-20 Comparison of Natural and Icing Wind Tunnel Ice Shapes. (See Reference R19.) R-33
R-21 Comparison of Drop Impingement and Ice Accretion Code Results With Experimental Ice Accretion Produced in the NASA IRT (V = 135.8 kts, TS = -15.8°C, LWC = 1.16 g/m3, MVD = 50.0 ȝm, icing duration = 517.1 s, GLC305-836-23 airfoil model chord = 0.9144 m). (See Reference R21.) R-35 R-22 Comparison of Drop Impingement and Ice Accretion Code Results With Experimental Ice Accretion Produced in the NASA IRT (V = 179.8 kts, TS = -15.6°C, LWC = 0.33 g/m3, MVD = 20.0 ȝm, icing duration = 1224.0 s, NLF0414-611 airfoil model chord = 0.9144 m). (See Reference R21.) R-36
xiii
R-23 Comparison of Drop Impingement and Ice Accretion Code Results With Experimental Ice Accretion Produced in the Boeing Research Aerodynamic and Icing Wind Tunnel (V = 130.2 kts, TS = -7.2°C, LWC = 1.0 g/m3,
MVD = 24.8 ȝm, icing duration = 1200.0 s, airfoil model chord = 0.914 m). (See Reference R21.) R-36
R-24 Comparison of Drop Impingement and Ice Accretion Code Results With Experimental Ice Accretion Produced in the Boeing Research Aerodynamic and Icing Wind Tunnel (V = 130.2 kts, T = -7.2°C, LWC = 1.0 g/m , 3 S MVD = 38.8 ȝm, icing duration = 1200.0 s, airfoil model chord = 0.914 m).
(See Reference R21.) R-37
S-1 Airplane Forces and Pitching Moments S-1
S-2 Aerodynamic Effects of Standard Roughness (0.011-inch Carborundum Grains) for a NACA 652-215 Airfoil. (The Carborundum grains were uniformly distributed at a density of 5 to 10 percent of the area that extended from the airfoil’s leading edge to 0.08c of the 24-inch model.) (See Reference S1.) S-2 xiv
List of Tables Table Page 1 Required Carburetor and Fuel System Ice Protection 2 2 Ice Protection Requirements for Aircraft not Certified for Operations in Known or Forecast Icing Conditions 3 3 Other 14 CFR Requirements Applicable for all Ice Protection Equipment and Systems 4 4 Additional Ice Protection Equipment and System Requirements for Aircraft Certified for Flight into Known and Forecast Icing Conditions 5 5 Means of Compliance for IPS Requirements 8 6 Continuous and Intermittent Maximum Icing Conditions 12 A-1 Related 14 CFR Parts 23, 25, 27, and 29 Regulations A-2 A-2 Related 14 CFR Part 33 – Airworthiness Standards: Aircraft Engines Regulations A-6 A-3 Related 14 CFR Part 35 – Airworthiness Standards: Propellers A-6 A-4 Related Operating Rules A-6 C-1 Chronology of 14 CFR Part 25 Icing Certification Requirements C-4 C-2 Chronology of 14 CFR Part 23 Icing Certification Requirements C-9 C-3 Chronology of 14 CFR Part 33 Engine Icing Certification Requirements C-11 I-1 Langmuir Distributions I-3 P-1 Measurable Definitions of Icing Intensity P-3 P-2 Suggested Data Sheet for Documenting Icing Exposures P-5 P-3 Effects on Aircraft P-6 P-4 Checklist of Items to be Evaluated in Icing Conditions P-7 R-1 Ranking of Ice Shape Evaluation Parameters R-39 R-2 Ranking of Water Catch Evaluation Parameters R-39 xv (and xvi)
1. PURPOSE.
a. This advisory circular (AC) tells type certificate and supplemental type certificate applicants how to comply with the ice protection requirements of Title 14 of the Code of Federal Regulations (14 CFR) parts 23, 25, 27, 29, 33, and 35. It also provides guidance for aircraft type certificate holders on how to maintain the aircraft’s airworthiness when operating in an icing environment. Applicants seeking an FAA type certificate for their aircraft, aircraft engines, or propellers, or a supplemental type certificate for an existing aircraft model or an aircraft component must adhere to ice protection regulations. You will find these requirements embedded throughout 14 CFR parts 23, 25, 27, 29, 33, and 35. (Ice protection regulations are listed in tables 1 through 4 in section 6 of this AC.) This AC also directs you to other ACs providing guidance for compliance with specific icing-related regulations.
b. This AC is not mandatory and does not constitute a regulation. It describes acceptable means, but not the only means to: (1) gain FAA approval of aircraft ice protection equipment and systems; (2) determine two-engine airplane airworthiness in icing conditions during Extended Range Operation with Two-engine Airplanes (ETOPS)); and (3) evaluate aircraft airworthiness following deicing and anti-icing before takeoff. It also provides guidance on operating aircraft in an icing environment that may affect the airworthiness of the aircraft. However, if you use the means described in this AC, you must follow the guidance in its entirety.
2. CANCELLATION. This AC cancels AC 20-73, Aircraft Ice Protection, dated April 21, 1971.
3. APPLICABILITY. This AC provides guidance to:
a. Applicants for approval of aircraft, aircraft engines, propellers, and airframe ice protection equipment and systems. This guidance applies to applicants seeking certification approval of their aircraft or aircraft engine for flight into known or forecast icing conditions and those applicants not seeking approval for flight in icing conditions. See appendix A of this AC for a list of 14 CFR parts 23, 25, 27, 29, 33, and 35 regulations applicable to aircraft, aircraft engines, and propeller icing certification.
b. FAA personnel accepting information that shows compliance.
c. Type certificate holders seeking ETOPS approval of their airplanes. Guidance provided in this AC addresses airframe ice accretion following a cabin depressurization or engine failure.
AC 120-42A, Extended Range Operation with Two-Engine Airplanes (ETOPS), provides additional information on the approval of extended range operations of two-engine airplanes.
d. Operators seeking assurance that their aircraft continues to comply with airworthiness requirements and operating rules that allow the use of deicing and anti-icing fluids to prevent ice, snow, and frost from adhering to the wing.
e. Applicants seeking approval under 14 CFR parts 23, 25, 27, or § 29.1419 for new type certificates, supplemental type certificates, and an amendment to existing type certificates for aircraft certificated under the Civil Aviation Regulations (CAR) parts 3 and 4b.
4. RELATED ADVISORY CIRCULARS, DOCUMENTS, READING MATERIAL, AND NOMENCLATURE.
See appendix A, sections A.2 and A.3, of this document, for additional icing approval information, related guidance material, and reading material. The FAA Aircraft Icing Handbook, DOT/FAA/CT-88/8-1, dated March 1991, provides comprehensive information on weather, icing physics, icing analysis, ice protection system design, ice detection systems, and certification of these systems. See also the Electronic Aircraft Icing Handbook (a subset of the Aircraft Icing Handbook) at: http://aar400.tc.faa.gov/Programs/FlightSafety/icing/eaihbk.htm. Appendix B of this AC defines terms, abbreviations, and symbols used in this document.
5. ICE PROTECTION REGULATORY DEVELOPMENT BACKGROUND.
Appendix C of this AC provides information for type design and supplemental type design applicants to determine the icing regulations that were applicable during the original type design certification of their aircraft. (See 14 CFR § 21.101 for icing requirements that apply to modified products.) Appendix C of this AC also provides background information on the historical development of aircraft, aircraft engine, and propeller icing regulatory requirements.
6. APPLICABLE AIRFRAME, ENGINE, AND PROPELLER ICE PROTECTION REQUIREMENTS.
a. Ice contamination on aircraft surfaces and components, aircraft engines, and propellers may cause unsafe operation of the aircraft. Type certificate applicants may seek approval for operating their aircraft in known or forecast icing conditions. However, if the applicant does not obtain approval, the applicant must protect the aircraft engine and specific aircraft components against ice ingestion and ice contamination during unintended exposure to atmospheric icing conditions. Sections 6.1, 6.2, and 6.3 of this AC discuss these requirements. (See appendix A of this AC for a more detailed listing of icing regulations.)
b. Applicants for certification of an aircraft and an aircraft engine must comply with certification requirements addressing ice accumulation that may form in aircraft engine carburetors, fuel tanks, and fuel lines systems, for all atmospheric conditions. Table 1 lists these requirements.
Table 1. Required Carburetor and Fuel System Ice Protection Affected Area 14 CFR part 23 14 CFR part 25 14 CFR part 27 14 CFR part 29 14 CFR part 33 Fuel tank and carburetor vapor vents § 23.975 § 25.975 § 27.975 § 29.975 Fuel tanks and fuel lines § 23.951(c) § 25.951(c) § 27.951(c) § 29.951(c) § 33.67(b)(4)(ii) Fuel strainers and filters § 23.997(e) Carburetor icing § 23.1093 § 23.1095 § 23.1097 § 23.1099 § 23.1101 § 23.1157 § 23.1189 § 25.1093 § 25.1101 § 25.1157 § 25.1189 § 27.1093 § 29.1093 § 29.1101 § 29.1157 § 29.1189 § 33.35(b) 2
6.1 Ice Protection Requirements: Unintended Exposure to Icing Conditions. For aircraft not certificated for operation in known or forecast icing conditions, airframe and aircraft engine type certificate applicants must provide ice protection for aircraft engines, the airframe, and airframe components to address unintended flight operations into icing conditions. (See table 2 below.) The exceptions to some of these requirements are those 14 CFR part 23 and 27 aircraft not certificated for instrument flight rule (IFR) operations. See appendix A, table A.1 of this AC for a detailed list of the applicable regulatory requirements.
Table 2. Ice Protection Requirements for Aircraft not Certified for Operations in Known or Forecast Icing Conditions Affected Area 14 CFR part 23 14 CFR part 25 14 CFR part 27 14 CFR part 29 14 CFR part 33 In-flight icing Appendix C of Appendix C Appendix C of 14 CFR Appendix C of 14 CFR conditions 14 CFR part 25 part 29 (AC 29-2C provides a 10,000-foot altitude-limited envelope for rotorcraft use. For rotorcraft limited to a flight envelope more restrictive than 14 CFR part 29, Appendix C, you may reduce the icing conditions used for approval of the ice protection equipment and systems to that of the limited flight envelope.)
part 29 (AC 29-2C provides a 10,000-foot altitude-limited envelope for rotorcraft use. For rotorcraft limited to a flight envelope more restrictive than 14 CFR part 29, Appendix C, you may reduce the icing conditions used for approval of the ice protection equipment and systems to that of the limited flight envelope.)
Aeroelastic stability (structural vibration and flutter) § 25.629(d)(3) Control system design § 23.685(a) § 25.685(a) § 27.685(a) § 29.685(a) Engine, engine § 23.901(d)(2) § 25.901(c) § 27.939 § 29.901(c) § 33.68 installation, and § 23.903 § 25.939 § 27.1093 § 29.939 § 33.77 air induction § 23.905 (e) § 25.941 § 27.1041 § 29.1041 (as systems for § 23.929 § 25.1093 § 29.1093 required engines and § 23.939 § 25.1093 § 29.1105 by 14 CFR engine § 23.975(a)(1) § 25.1105 part 25) accessories § 23.1093 cooling § 23.1105 Indication of powerplant IPS or fuel system(s) heater operation installed to prevent ice § 23.1305(c)(7) § 25.1305(c)(5) § 27.1305(p) § 29.1305(a)(17) Air data systems § 23.1323(d) § 23.1325 § 23.1326 § 25.1323(i) § 25.1325(b) § 25.1326 § 27.1325(b) § 29.1323(f) § 29.1325(c) IPS = Ice protection system 3
Table 3 lists 14 CFR regulations that apply to ice protection equipment and systems.
Table 3. Other 14 CFR Requirements Applicable for all Ice Protection Equipment and Systems Affected Area 14 CFR part 23 14 CFR part 25 14 CFR part 27 14 CFR part 29 14 CFR part 33 14 CFR part 35 Ice protection equipment § 23.859(i)(2) § 25.859(i)(2) § 27.859(k)(2) § 29.859(i)(2) and systems design and § 23.601 § 25.601 § 27.601 § 29 .601 construction § 23.603 § 25.603 § 27.603 § 29.603 § 23.605 § 25.605 § 27.605 § 29.605 § 23.607 § 25.607 § 27.607 § 29.607 § 23.609 § 25.609 § 27.609 § 29.609 § 23.611 § 25.611 § 27.611 § 29.611 § 23.613 § 25.613 § 27.613 § 29.613 Influence of the IPS on § 23.863 § 25 .863 § 27 .863 § 29 .863 operation of other § 23.1013(d)(2) § 25.1013(d)(2) § 27.1307(d)(2) § 29.1013(d)(2) systems, components, § 23.1199(b) § 25.1199(b) § 27.1309 § 29.1199(b) and requirements (e.g., § 23.1309 § 25.1307(c) § 27 .1327 § 29.1309 flammability protection § 23.1307 § 25.1309 § 27.1351 § 29.1351 and strong electrical § 23.1327 § 25 .1327 § 29.1353 fields and § 23.1351 § 25.1351 electromagnetic § 23.1357 § 25.1353 interference effects produced by the IPS).
§ 25.1357 Fuel tank ignition prevention § 25.981 IPS equipment proper function and installation § 23.1301 § 23.1309 § 25.1301 § 25.1309 § 27.1301 § 27.1309 § 29.1301 § 29.1309 Miscellaneous equipment § 25.1455 Pilot information and § 23.1525 § 25.1525 § 27.1525 § 29 .1525 operating limitations. § 23.1559(b) § 25.1583 § 27.1559 § 29 .1559 § 23.1583(h) § 25.1585 § 27.1583 § 29.1583 § 23.1585 § 27.1585 § 29.1585 Information on continuing airworthiness of the IPS (may include IPS inspection and maintenance information).
§ 23.1529 § 25.1529 § 27.1529 § 29.1529 § 33.4 § 35 .4
6.2 Ice Protection Requirements: Intentional Operations in Known and Forecast Icing Conditions. Aircraft engine and airframe type certificate and supplemental type certificate applicants must provide ice protection for aircraft engine, airframe, and airframe components to ensure the aircraft and aircraft engines operate safely in known or forecast icing conditions.
Table 4 contains ice protection requirements for aircraft certificated for flight into known or forecast icing conditions. You must comply with the requirements in table 4 below and those requirements in section 6.3 of this AC.
Table 4. Additional Ice Protection Equipment and System Requirements for Aircraft Certificated for Flight into Known and Forecast Icing Conditions Affected Area 14 CFR part 23 14 CFR part 25 14 CFR part 27 14 CFR part 29 14 CFR part 33 Aeroelastic stability (structural vibration and flutter) § 23.629 § 27.629 § 29.629 Windshield and pilot compartment view § 23.775 § 25.773(b)(1)(ii) § 25.775(d) § 29.773(b)(1)(ii) Engines and their installation (reevaluated § 23.901(d)(2) § 25.903 § 27.1093 § 29.1093 § 33.65 for ice ingestion resulting from extended § 23.903 § 25.1093 § 33.77 exposures to icing conditions), and air induction systems for engines and engine accessories cooling § 23.1093 Acceptable engine operability with bleed § 33.66 air required for ice protection, engine ice § 33.68 protection, and indication of engine ice protection operation.
§ 33.89(b) Propellers § 23.905(e) § 23.929 § 25.929 Airframe icing and safe in-flight operations in icing conditions § 23.1416 § 23.1419 § 25.1403 § 25.1419 § 27.1419 § 29.1419 Air data system § 23.1323(d) § 23.1325 § 23.1326
6.3 Ice Protection Requirements: Operating Safely in Icing Conditions. In order for airplanes certificated to 14 CFR part 23 to operate as safely as possible in icing conditions, applicants must demonstrate their airplane’s performance, controllability, maneuverability, and stability is at least equal to the requirements of 14 CFR part 23, Subpart B, as defined in 14 CFR § 23.1419(a). (For additional guidance, see AC 23-8B, AC 23.143-1, and AC 23.1419-2C.) To be able to operate as safely as possible in icing conditions, compliance with specific sections of 14 CFR part 25, Subpart B must be demonstrated. Acceptable means of showing compliance with the certification requirements for flight operations into icing conditions may be found in AC 25-7A and AC 25.1419-1A. Acceptable means of showing compliance with the certification requirements for rotorcraft flight operations into icing conditions may be found in AC 27-1B and AC 29-2C.
7. CERTIFICATION PROCEDURES.
7.1 Certification Procedures: General. At the beginning of the type certification process, applicants should coordinate their icing certification plan with their aircraft certification office (ACO) or engine certification office (ECO). Appendix D of this AC contains typical ice protection equipment design and certification flow charts. Also, when seeking approval for a modified product, you need to comply with 14 CFR § 21.101, commonly known as the “Change Product Rule.” You may choose to use AC 21.101-1 as a means of showing compliance with 14 CFR § 21.101.
You should present your certification plan (checklist) for FAA approval at the start of the IPS design and development efforts. Your certification plan should describe all planned tests and evaluations intended to lead to the certification of your IPS. Further, your certification plan should identify specific items to be certificated, relevant certification requirements, and proposed methods of compliance. Your certification plan should clearly identify analyses and tests, or references to similarity of designs, you intend to use for the certification of the IPS. To help you save time and certification cost, it is important to get FAA concurrence before conducting certification tests. At a minimum, your certification plan should include:
a. An aircraft or aircraft engine systems description.
b. The IPS description.
c. A detailed description of aircraft IPS changes.
d. The certification basis for the requested approval.
e. The certification checklist, including the names of specific designated engineering representatives (DER). The checklist should also include the reports documenting compliance with the regulations.
f. Certification schedules for both the applicant and the FAA.
g. Description of existing analyses or tests.
h. Conformity inspection plans, including the conformity demonstration location.
i. Hazard assessments.
j. Software considerations.
k. High intensity radiated fields and lightning considerations.
l. A list of icing certification test results, if complete, requiring special operating procedures.
m. If the ice protection or detection systems contain complex electronic hardware (such as programmable logic devices (PLD) or application specific integrated circuits (ASIC)), provide plans showing the level of design assurance of these devices. The level of design assurance should be equal to the PLD or ASIC’s potential contribution to aircraft hazards and system failures that could result from electronic hardware faults or malfunctions.
7.2 Certification Procedures: Airframe Manufacturer.
a. Applicants for part 23 type certificates should follow the IPS certification procedures described in AC 23-8B and AC 23.1419-2C. See AC 25-7A and AC 25.1419-1A for similar guidance for 14 CFR part 25 type certificated applicants. Rotorcraft applicants should follow the 6
guidance in AC 27-1B and AC 29-2C. Supplemental type certificate applicants should also follow the guidance in AC 21-40.
b. Airframe type certificate applicants should present their ACO with a certification plan and checklist showing compliance with the applicable regulatory requirements. This plan should include documented evidence of compliance with the certification requirements. Your design analysis should identify the critical design points and predict the IPS performance for the icing conditions defined in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. Present your test proposals to the ACO for their approval before testing. As a reminder, you are responsible for showing compliance with applicable 14 CFR parts 23, 25, 27, or 29 regulations, including those covering the aircraft engine and propeller.
7.3 Certification Procedures: Aircraft Engine Manufacturer.
a. Aircraft engine type certificate applicants should review the regulatory guidance material in AC 20-147 and AC 33-2B, and submit their icing certification plan and checklist to their ECO for acceptance. Those applicants seeking supplemental type certification for their aircraft engines should consult AC 21-40 for guidance material. Aircraft engine manufacturers should present a design analysis showing enough critical design points to ensure the engine will function adequately in icing conditions described in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. Your design analysis must also address freezing fog, falling snow, and blowing snow conditions. See 14 CFR §§ 23.1093, 25.1093, 27.1093, and 29.1093. Aircraft engine type certificate applicants should consider guidance provided in this AC when performing the engine ice protection system design analysis. You should present your test proposal and test procedures to the FAA and get approval from the FAA before testing.
NOTE: 14 CFR part 33 tests are typically conducted in icing wind tunnels or on outdoor test stands to simulate in-flight icing envelopes defined in 14 CFR parts 25, Appendix C and 14 CFR part 29, Appendix C. Compliance with 14 CFR §§ 23.1093(b) or 25.1093(b) tests is typically shown in either natural icing conditions or in simulated icing conditions. Simulated icing conditions may include flight behind an icing tanker or testing in climatic test chambers.
b. Your test proposal should include enough data points throughout the engine power or thrust range to show the engine operates satisfactorily under the proposed test conditions.
8. COMPLIANCE MEANS. Airframe type certificate applicants must perform an analysis and testing to show the acceptability of the aircraft’s ice protection system. See 14 CFR §§ 23.1419, 25.1419, 27.1419, and 29.1419. You should address all flight configurations of the aircraft. To verify your analysis and icing anomalies, and to show the IPS and associated components are effective for safe operations in icing conditions, you must flight test the aircraft or its components in measured natural atmospheric icing conditions. With prior ACO approval, you may use laboratory dry air or simulated icing tests, dry air flight tests, or flight tests in measured simulated icing conditions to supplement flight tests in measured natural icing conditions. The ACs listed in table 5 provide guidance on means of compliance with the FAA regulations pertaining to IPS certification. For further guidance on supplemental type certificates 7
and amended type certificates, see sections 8.5 and 10.2 of this AC.
Table 5. Means of Compliance for IPS Requirements Advisory Circular 14 CFR Regulation Advisory Circular 14 CFR Regulation Advisory Circular 14 CFR Regulation AC 20-147 § 23.901(d)(2) AC 23.1309-1C § 23.1309 AC 25.1419-1A § 25.773 § 23.903 § 25.929 § 23.1093 § 25.975 § 25.903 § 25.1093 § 25.1093 § 25.1323 § 27.1093 § 25.1325 § 29.1093 § 25 .1326 § 33.68 § 25.1403 § 33.77 § 25.1419 § 25.1419 (safe operation in icing conditions – 14 CFR part 25, Subpart B) AC 23-8B and § 23.1419 (safe AC 23.1419-2C § 23.773 AC 27-1B §27.629 AC 23-143.1 operation in icing conditions – 14 CFR part 23, Subpart B) § 23.775 § 23.929 § 23.975 § 23.1093 § 23.1309 § 23.1323 § 23.1325 § 23.1419 § 23.1419 (safe operation in icing conditions – 14 CFR part 23, Subpart B) §27.975 §27.1093 §27.1301 §27.1309 §27.1325 §27.1419 §27.1419 (safe operations in icing conditions – 14 CFR part 27, Subpart B) AC 23-16A § 23.901(d)(2) AC 25-7A § 25.1419 (safe AC 29-2C § 29.629 § 23.905(e) operation in icing § 29.773 § 23.929 conditions – 14 CFR § 29.975 § 23.975 part 25, Subpart B) § 29.1093 § 23.1093 § 29.1301 § 29.1309 § 29.1325 § 29.1419 § 29.1419 (safe operation in icing conditions – 14 CFR part 29, Subpart B) § 29.1323 AC 23-17A § 23.1301 § 23.1323 § 23.1326 AC 25.629-1A § 25.629 AC 33-2B § 33.75 AC 23.629-1B § 23.629 AC 25.1309-1A § 25.1309 8